1. Field of the Invention
This invention relates to an ion milling machine and more particularly to an ion milling machine specimen elevator and airlock mechanism which facilitates specimen viewing and specimen exchange.
2. Description of the Prior Art
Ion milling machines are used in the preparation of electron microscope specimens. An article entitled "An Improved Ion Thinning Apparatus" by A. H. Heuer, et al. from The Review of Scientific Instruments, Vol. 42, No. 8, Aug. 1971, pp. 1177-1184, which is herein incorporated by reference, provides good background information on ion milling.
Ion milling machines are employed for thinning specimens to a thickness of the order of 0.5 micrometers for examination by transmission electron microscopy. Desired thinning is accomplished by placing the specimen in the path of one or more beams of energetic ions and neutral atoms which sputter atoms from the specimen surface. Although material removal is very slow during ion milling, this procedure does much less damage to the underlying specimen than other material removal methods such as cutting or grinding. Ion milling has become widely used as a method for electron microscope specimen preparation, especially for those materials that because of their chemical nature cannot be thinned by electro-polishing or chemical polishing methods.
Ion thinning is accomplished in an evacuated chamber; usually the specimen is rotated during exposure to the ion beams to improve the uniformity of thinning across the specimen surface. In order to study the progress of the ion milling operation, the specimen must be periodically examined using a lower power microscope. This inspection is difficult in prior art ion thinning machines because material sputtered from the specimen and specimen holders coats the observation windows and sources of illumination, thereby obscuring the view of the specimen. Further, because the specimen is positioned near the center of the work chamber and is surrounded by the ion guns and other related mechanisms, the specimen must be viewed at an uncomfortably long distance.
A second deficiency of presently available ion thinning units is that the entire work chamber must be raised to atmospheric pressure before specimens can be exchanged. Raising the work chamber to atmospheric pressure has at least three disadvantages: (1) the time for specimen exchange is lengthy because a large volume of air has to be evacuated before the ion guns can be turned on; (2) the ion guns are initially less stable after exposure to atmospheric pressure and must be pumped for a long period of time before they restabilize; and (3) a costly valving mechanism is needed to isolate the work chamber from the pumping system.
In presently available ion thinning units, it is awkward to load and unload specimens. This is disadvantageous because electron microscope specimens are extremely fragile and are easily damaged by the small mechanical shocks they receive during the loading and unloading operation.